Thin film transistor and display device

ABSTRACT

Provided is a thin film transistor comprising an oxide semiconductor thin film layer and has a threshold voltage that does not change much due to light, a bias stress or the like, thereby exhibiting excellent stress stability. A thin film transistor of the present invention is provided with: a gate electrode; two or more oxide semiconductor layers that are used as a channel layer; an etch stopper layer for protecting the surfaces of the oxide semiconductor layers; a source-drain electrode; and a gate insulator film interposed between the gate electrode and the channel layer. The metal elements constituting an oxide semiconductor layer that is in direct contact with the gate insulator film are In, Zn and Sn. The hydrogen concentration in the gate insulator film, which is in direct contact with the oxide semiconductor layer, is controlled to 4 atomic % or less.

TECHNICAL FIELD

The present invention relates to a thin-film transistor (TFT) to be usedin display devices such as liquid crystal displays and organic ELdisplays; and a display device having the thin-film transistor.

BACKGROUND ART

As compared with widely used amorphous silicon (a-Si), amorphous(non-crystalline), oxide semiconductors have high carrier mobility (alsocalled as field-effect mobility, which may hereinafter be referred tosimply as “mobility”), a wide optical band gap, and film formability atlow temperatures, and therefore, have highly been expected to be appliedfor next generation displays, which are required to have large sizes,high resolution, and high-speed drives; resin substrates having low heatresistance; and others (see Patent Document 1).

Among the oxide semiconductors, amorphous oxide semiconductorsconsisting of indium, gallium, zinc and oxygen (In—Ga—Zn—O, which mayhereinafter be referred to as “IGZO”) have preferably been used inparticular because of their very high carrier mobility. For example,non-patent literature documents 1 and 2 disclose thin film transistors(TFTs) in which a thin film of an oxide semiconductor having an In:Ga:Znratio equal to 1.1:1.1:0.9 (atomic % ratio) was used as a semiconductorlayer (active layer).

When an oxide semiconductor is used as a semiconductor layer of a thinfilm transistor, the oxide semiconductor is required to have a highcarrier concentration and a high mobility and excellent TFT switchingproperties (transistor characteristics or TFT characteristics).Specifically, the oxide semiconductor is required to have (1) a highon-state current (i.e., the maximum drain current when a positivevoltage is applied to both a gate electrode and a drain electrode); (2)a low off-state current (i.e., a drain current when a negative voltageis applied to the gate electrode and a positive voltage is applied tothe drain voltage, respectively); (3) a low SS value (SubthresholdSwing, i.e., a gate voltage needed to increase the drain current by onedigit); (4) a stable threshold value (i.e., a voltage at which the draincurrent starts to flow when a positive voltage is applied to the drainelectrode and either a positive voltage or a negative voltage is appliedto the gate voltage, which voltage may also be called as a thresholdvoltage) showing no change with time (which means that the thresholdvoltage is uniform in the substrate surface); and (5) a high mobility.

Furthermore, TFTs using an oxide semiconductor layer such as IGZO arerequired to have excellent resistance to stress such as voltageapplication and light irradiation (stress stability). It is pointed outthat, for example, when a voltage is continuously applied to the gateelectrode or when light in a blue emitting band in which lightabsorption arises is continuously irradiated, electric charges aretrapped on the boundary between the gate insulator film and thesemiconductor layer of a thin film transistor, which induces a largeshift of the threshold voltage toward negative side due to the change ofelectric charges within the semiconductor layer, resulting in avariation of switching characteristics. When a thin film transistor isused, such variation of the switching characteristics due to the stressby the voltage application and the light irradiation causesdeterioration of reliability in a display devices itself.

Similarly in an organic EL display panel, the semiconductor layer isirradiated by light leaked out from a light emission layer, causingproblems as a variation and a deviation of the threshold voltage in theTFT.

Such a shift of threshold voltage of the TFT particularly deterioratesthe reliability of display devices such as a liquid crystal display andan organic EL display. Therefore, an improvement in the stress stability(a small variation before and after the stress tests) is eagerlydesired.

Patent Document 2 is named as an example which improved electricalproperties of TFT. The document discloses a technology to lower thehydrogen concentration to smaller than 6×10²⁰ atoms/cm³ in an insulatingfilm, including a gate insulator film, which is in direct contact to anoxide semiconductor layer of a channel region and to suppress diffusionof hydrogen into the oxide semiconductor layer. Diffusion of hydrogeninduces excess carrier concentration in the oxide semiconductor layerand negative shift of the threshold voltage, turning the transistornormally-on state in which the drain current flows even without puttingthe gate bias (V_(g)=0 V), which makes the transistor faulty. The PatentDocument 2 thereby describes that diffusion of hydrogen into the oxidesemiconductor layer was suppressed by employing a hydrogen-reduced oxideinsulating film for the insulating film which is in direct contact tothe oxide semiconductor layer. The document also explains that theelectrical properties of a transistor are improved because oxygen isprovided from the oxide insulating film to oxygen related defects in theoxide semiconductor layer. Furthermore, according to the Patent Document2, it is necessary to decrease the hydrogen concentration to smallerthan 6×10²⁰ atoms/cm³ in the insulating film in order to secure theeffect. It is also stated vital to select and use hydrogen-free gas asthe source gas in the process of plasma CVD of the hydrogen-reducedinsulating film. In the Patent Document 2, SiF₄ is employed for thesource gas instead of generally-used SiH₄. However, no attention is paidto improving stress stability, particularly decreasing the thresholdvoltage shift by light and electrical biasing stresses.

PRIOR ART DOCUMENTS Patent Document

-   Patent Document 1: Japanese Patent Laid-open Publication No.    2011-108873-   Patent Document 2: Japanese Patent Laid-open Publication No.    2012-9845

Non-Patent Literature Document

-   Non-patent Literature Document 1: Kotaibutsuri (Solid State    Physics), Vol. 44, p. 621 (2009)-   Non-patent Literature Document 2: Nature, Vol. 432, p. 488 (2004)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention has been completed under the circumstancesdescribed above, and an object of the present invention is to provide athin film transistor comprising an oxide semiconductor layer, havinghigh stability to light and biasing stresses to have a small variationof threshold voltage, and a display device having the thin filmtransistor.

Means for Solving the Problems

The thin film transistor of the present invention capable to solve theproblem, comprises a gate electrode, an oxide semiconductor layercomprising two or more layers configured to be used for a channel layer,an etch stopper layer configured to protect the surface of the oxidesemiconductor layer, a source-drain electrode, a gate insulator filminterposed between the gate electrode and the channel layer. Metalelements constituting the oxide semiconductor layer which is in directcontact to the gate insulator film are In, Zn, and Sn. Hydrogenconcentration is also controlled down to 4 atomic % or smaller in thegate insulator layer which is in direct contact to the oxidesemiconductor layer.

In a preferred embodiment of the present invention, the gate insulatorfilm is a single layer structure or a laminate structure consisting twoor more layers. In a case of the laminate structure, hydrogenconcentration is controlled to 4 atomic % or less in a layer which is indirect contact to the oxide semiconductor layer.

In a preferred embodiment of the present invention, the oxidesemiconductor layer in direct contact to the gate insulator filmsatisfies the requirements represented by expressions shown below,wherein [In], [Zn], and [Sn] represent the content (in atomic %) of theelements each relative to the total content of all the metal elementsother than oxygen in the oxide semiconductor layer;

-   -   15≦[In]≦35, 50≦[Zn]≦60, 15≦[Sn]≦30        and the oxide semiconductor layer in direct contact to the        source-drain electrode satisfies the requirements represented by        expressions shown below, wherein [In], [Zn], [Sn], and [Ga]        represent the content (in atomic %) of the elements each        relative to the total content of all the metal elements other        than oxygen in the oxide semiconductor layer;    -   10≦[In]≦20, 30≦[Zn]≦40, 5≦[Sn]≦15, and 35≦[Ga]≦50.

The present invention further encompasses a display device having atleast one of the thin film transistors described above.

Effects of the Invention

The present invention can provide a thin film transistor havingexcellent switching characteristics and stress stability shown by asmall variation of threshold voltage before and after applying the lightirradiation and the electrically negative biasing as well as a smallvariation of threshold voltage before and after applying theelectrically negative biasing. This is presumably because hydrogenconcentration is lowered to an appropriate range in the gate insulatorfilm which is in direct contact to the oxide semiconductor. A displaydevice of high reliability can be provided by employing the thin filmtransistor of the present invention.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic cross-sectional view for explaining the thin filmtransistor of the present invention.

MODE FOR CARRYING OUT THE INVENTION

The present inventors have made various studies to provide a thin filmtransistor comprising a TFT having an oxide semiconductor active layerconstituted of specific metal elements, which has excellent stressstability shown by a small variation of threshold voltage before andafter applying the light irradiation and the electrically negativebiasing as well as a small variation of threshold voltage before andafter applying the electrically negative biasing. As a result, thepresent inventors have found that an intended object can be achieved bylowering the hydrogen concentration in the gate insulator film in directcontact to the oxide semiconductor film to an appropriate range. Thepresent inventors further found that the gate insulator film in directcontact to the oxide semiconductor film can be formed at least byappropriately control deposition conditions such as for example thetemperature, plasma power density, flow rate ratio of SiH₄ to N₂O in thesource gas, of the plasma CVD process, thereby completing the presentinvention.

The thin film transistor of the present invention comprises a gateelectrode, an oxide semiconductor laminate comprising two or more layersand configured to be used for a channel layer, an etch stopper layerconfigured to protect the surface of the oxide semiconductor, asource-drain electrode (occasionally referred to “S/D electrode”), agate insulator film interposed between the gate electrode and thechannel layer. Metal elements constituting the oxide semiconductor layerwhich is in direct contact to the gate insulator film are In, Zn, andSn. Hydrogen concentration is also controlled to 4 atomic % or smallerin the gate insulator layer which is in direct contact to the oxidesemiconductor layer.

In the present specification, [In], [Zn], [Sn], and [Ga] represent thecontent (in atomic %) of each of the elements relative to the totalcontent of all the metal elements (In, Zn, Sn, and Ga) other than oxygen(O).

In the present specification, the wording “excellent in stressstability” means that a thin film transistor satisfies each of thefollowing requirements when it is subjected to (A) a negative biastemperature stress (NBTS) test in which a negative bias stress isapplied onto the gate electrode for a period of 2 hours, and (B) a lightand negative bias temperature stress (LNBTS) test in which white lightis irradiated to the TFT while negative bias stress is applied onto thegate electrode for a period of 2 hours;

-   -   (A) the threshold voltage shift (the absolute value of ΔV_(th))        is smaller than 5.0 V, after the NBTS test,    -   (B) the threshold voltage shift (the absolute value of ΔV_(th))        is smaller than 5.0 V, the SS value is smaller than 0.55        V/decade, and decrease of the on-current (the absolute value of        ΔI_(on)) is smaller than 10%, after the LNBTS test.

Measurement methods of these properties are described in detail later inExamples in the specification.

In the Patent Document 2 also discloses an invention trying to improvethe electrical properties by lowering hydrogen concentration in the gateinsulator film. However, it is different from the present invention inthe following aspects.

Firstly, there is no description on improvement of stress stability inthe Patent Document 2 which refers to the threshold voltage. On thecontrary, the problem to be solved in the present invention is providinga thin film transistor which is excellent in stress stability, showingsmall variation in the threshold voltage before and after the stressbiasing, as described above. According to studies conducted by thepresent inventors, it was elucidated that the negative bias temperaturestress (NBTS) stability is improved by lowering the amount of hydrogencontained in the gate insulator film. It was found further that thelight and negative bias temperature stress (LNBTS) stability is alsoimproved by lowering the amount of hydrogen contained in the gateinsulator film. These findings are not described in the Patent Document2.

Strictly speaking, the present invention and the Patent Document 2 aredifferent from each other in terms of the range of hydrogenconcentration in the gate insulator film. The difference is arising fromthe different methods of forming the gate insulator films as describedin detail later in the specification. As explained above, the PatentDocument 2 significantly decreased the hydrogen concentration in thegate insulator film down to smaller than 6×10²⁰ atoms/cm³ (=0.667 atomic%) by employing rarely-used SiF₄ instead of SiH₄ which is generallyused. The present invention, on the other hand, employs generally-usedSiH₄ for the source gas for the deposition of the gate insulator film.The hydrogen concentration in the gate insulator film is lowered to 4atomic % or smaller by appropriately controlling the gas flow rateratio, the deposition temperature, the plasma power density, and so on.In fact, extreme suppression of hydrogen as in the Patent Document 2 isnot appropriate because it induces excessively high depositiontemperature, excessively high plasma power density, and extremely lowdeposition rate of the gate insulator film, causing increase in takttime in the course of fabrication process of TFT. From the practicalpoint of view, the lower limit of the hydrogen concentration in the gateinsulator film is preferably 0.667 atomic % or more which is more thanthe upper limit of the hydrogen concentration in the Patent Document 2.

Hereinbelow, by referring to FIG. 1, the thin film transistor (TFT) ofthe present invention and its preferred fabrication method re describedin detail. FIG. 1 is a schematic cross sectional view for explaining oneexample of preferred embodiments of the present invention, but it is notintended that the present invention be limited thereto. FIG. 1, forexample, shows a TFT structure of a bottom gate type; however, TFTs arenot limited thereto, and TFTs may be those of a top gate type, having agate insulator film and a gate electrode successively from the side ofthe substrate on an oxide semiconductor layer.

As shown in FIG. 1, a gate electrode 2 and a gate insulator film 3 areformed on the substrate 1, and an oxide semiconductor layer 4 is formedthereon in the TFT of the present embodiment. On the oxide semiconductorlayer 4, a source-drain electrode 5 is formed, and a passivation film(insulating film) 6 is formed further thereon. A transparent conductivefilm is electrically connected to the drain electrode 5 through acontact hole 7. On the oxide semiconductor layer 4, an etch stopperlayer 9 is formed to protect the surface of the oxide semiconductorlayer 4.

First, a substrate is prepared. The kind of the substrate 1 is notparticularly limited, and there can be used those which have widely beenused in the field of display apparatus. An alkaline-free glass, a sodalime glass, or the like are exemplified. Among these, an alkaline-freeglass is preferably used.

Then, a gate electrode 2 is formed on the substrate 1. The kind of thegate electrode 2 is not particularly limited, and there can be usedthose which have widely been used in the field of the present invention.Specifically, metals of low electrical resistivity such as Al and Cu,and refractory metals of high heat resistance such as Mo, Cr, and Ti,and their alloys, can preferably be used for the gate electrode. Amethod of forming the gate electrode 2 is not particularly limited, andany of the methods usually used can be employed.

Next, a gate insulator film 3 is formed. The gate insulator film 3 isinterposed between the gate electrode 2 and the oxide semiconductor 4which is configured to be used for a channel layer. The presentinvention is characterized in that hydrogen concentration is regulatedto 4 atomic % or lower in the gate insulator film 3 which is in directcontact to the oxide semiconductor layer. It was elucidated byexperimental results obtained by the present inventors that resistancesto electrical biasing stress as well as stress by light irradiation andelectrically negative biasing are remarkably improved by controlling thehydrogen content in the gate insulator film 3 which was in directcontact to the oxide semiconductor layer 4, as described in Examplesbelow.

The gate insulator film 3 may consist of either a single layer or alaminate composed of two or more layers. The number of layers in thelaminate structure is not particularly limited. However, considering theproductivity and workability, it is preferably 3 or less.

When the gate insulator film 3 comprise a laminate structure, hydrogenconcentration is to be controlled to 4 atomic % or lower in a layerwhich is in direct contact to an oxide semiconductor layer 4. Hydrogenconcentration in a layer which is not in direct contact to the oxidesemiconductor layer 4 is not particularly limited.

From the point of view to improving the stress stability, the lowerhydrogen concentration in the gate insulator film 3, more preferable. Itis preferably lower than or equal to 3.5 atomic %, and more preferablylower than or equal to 3 atomic %. The lower limit of the hydrogenconcentration in the gate insulator film 3 is not particularly limitedfrom the point of view on the properties. However, considering themethod of forming the gate insulator film 3 explained later in thespecification, it is preferably higher than 0.667 atomic % which is theupper limit of the Patent Document 2.

In the present invention, the hydrogen concentration in the gateinsulator film can be decreased to the predetermined range byappropriately controlling the deposition condition of the plasma CVDmethod.

Specifically, deposition temperature is preferably controlled to about250° C. or higher. When the deposition temperature is lower than 250°C., the hydrogen concentration is not sufficiently decreased, resultingin deterioration of the stress stability as demonstrated in Examplesdescribed later in the present specification. It is deduced because thedensity of the insulating film is decreased by the decrease of thedeposition temperature, which increases Si—H bonding in the SiO₂ film.The deposition temperature is preferably higher than or equal to 270°C., and more preferably higher than or equal to 300° C. On the otherhand, the upper limit of the deposition temperature is preferablycontrolled to about 450° C. or lower considering application temperaturerange of the apparatus used for the film formation.

The plasma power density for the deposition is preferably controlled tolarger than or equal to roughly 0.6 W/cm². When the plasma power densityfor the deposition is lower than about 0.6 W/cm², the hydrogenconcentration is not sufficiently decreased, resulting in deteriorationof the stress stability as demonstrated in Examples described later inthe present specification. It is deduced because the density of theinsulating film is decreased by the decrease of the plasma powerdensity, which increases Si—H bonding in the SiO₂ film. The plasma powerdensity is preferably higher than or equal to 0.66 W/cm², and morepreferably higher than or equal to 0.7 W/cm².

It is also preferable to control the ratio of SiH₄ to N₂O as low aspossible in the gas mixture for the film deposition. In other words, theflow rate ratio (volume ratio) represented by SiH₄/N₂O is preferablycontrolled to a predetermined value or smaller. It is observed that thedensity of the SiO₂ film decreases when the flow rate ratio is high. Itis considered that such an insulating film contains a large number ofSi—H bonding.

Deposition conditions other than those described above are notparticularly limited, and any of the conditions usually used can beemployed.

Gas pressure, for example, is preferably controlled to about 50 to 300Pa in order to secure stable discharge.

The gate insulator film 3 formed by the above-described method mainlycomprises silicon oxide (SiO₂). In the meanwhile, Si—N bonding may beincluded as long as hydrogen content in the gate insulator film is notincreased. For example, a dense silicon oxide film (SiO₂) having lowhydrogen concentration exhibits fine insulating characteristics.However, its deposition rate is likely to be small. As such, bycombining a SiO_(x) film with a SiN_(x) film formed at a relatively highdeposition rate to constitute a laminate gate insulator film 3, both theinsulating property and the productivity can be satisfied. In order tosecure the insulating property, thickness of the SiN_(x) film ispreferably equal to 50 times or smaller than that of the SiO_(x) film,and more preferably 25 times or smaller.

Subsequently, the oxide semiconductor layer 4 is formed on top of thegate insulator film 3. The oxide semiconductor layer 4 is generallyinterposed between the gate insulator film 3 and a source-drainelectrode (S/D electrode) 5 in a thin film transistor. In the presentinvention, metal elements in the oxide semiconductor layer which is indirect contact to the gate insulator film 3 (sometimes referred to“oxide semiconductor on the side of the gate insulator film”hereinafter) are consisting In, Zn, and Sn, that is the oxidesemiconductor layer is IZTO.

Effects of the metal elements are roughly as described below.

In increases the carrier density and enhances the mobility in the oxidesemiconductor. It is noted, however, that excessive content of In turnsthe semiconductor to a conductor by generating excessive carriers anddeteriorates the stability to the stresses.

Sn is an element effective to improve the chemical resistance such aswet etching resistance of the oxide semiconductor. However, etchingworkability is deteriorated when the amount of Sn is increased in thefilm.

It is considered that Zn contributes to stabilization of the amorphousstructure. Zn contributes to the improvement of the stability of stressresistance. Excessive amount of Zn in the oxide, however, is liable tocause crystallization of the film, and occurrence of residues in theetching process.

The oxide semiconductor layer 4 may consist of a laminate composed oftwo or more layers. The number of layers in the laminate structure isnot particularly limited. However, considering the productivity andworkability, it is preferably three or less, and more preferably two.

The kind of metal elements constituting each of the layers other thanthe oxide semiconductor layer on the side of the gate insulator film isnot particularly limited. The layers other than the oxide semiconductorlayer on the side of the gate insulator film preferably contain In, Zn,and Sn.

The oxide semiconductor layer which is in direct contact to thesource-drain electrode (S/D electrode) 5 (sometimes referred to “oxidesemiconductor on the side of S/D electrode” hereinafter) are preferablycomprising In, Zn, Sn, and Ga, which significantly improves the stressstability.

Ga is an element effective to suppress generation of oxygen deficiency,to stabilize the amorphous structure, and to improve the stressstability, particularly the stability to light and negative biasstresses, of an oxide semiconductor layer. It is noted, however, themobility is lowered when the amount of Ga is increased in the film.

It is preferable to control the ratio of metal elements constituting theoxide semiconductor layer 4 (contents (in atomic %) of respective metalelements relative to the total amount of all the metal elements otherthan oxygen) to an appropriate range depending so that favorable TFTcharacteristics are secured.

Specifically, the metal elements constituting the oxide semiconductorlayer on the side of the gate insulator film preferably satisfy therequirements represented by expressions shown below, wherein [In], [Zn],and [Sn] represent the content (in atomic %) of the elements eachrelative to the total content of all the metal elements other thanoxygen in the oxide semiconductor layer. By satisfying the requirements,the desirable effect of each element mentioned above can be effectivelyexerted.

-   -   15≦[In]≦35, more preferably 15≦[In]≦25,    -   50≦[Zn]≦60,    -   15≦[Sn]≦30

Further, the metal elements constituting the oxide semiconductor on theside of S/D electrode preferably satisfy the requirements represented byexpressions shown below, wherein [In], [Zn], [Sn], and [Ga] representthe content (in atomic %) of the elements each relative to the totalcontent of all the metal elements other than oxygen in the oxidesemiconductor layer. By satisfying the requirements, the desirableeffect of each element mentioned above can be effectively exerted.

-   -   10≦[In]≦20    -   30≦[Zn]≦40    -   5≦[Sn]≦15    -   35≦[Ga]≦50

The thickness of the oxide semiconductor layer 4 is preferably about 10nm or larger, and 200 nm or smaller.

The oxide semiconductor layer 4 is preferably formed by DC sputtering orRF sputtering method using a sputtering target having the samecomposition as that of the thin film. Alternatively, the film formationmay be carried out by a co-sputtering method, in which multiple targetswith different compositions are used.

After wet etching of the oxide semiconductor layer 4, it is subjected topatterning. Immediately after the patterning, a pre-annealing heattreatment may be conducted for the purpose of improvement of filmquality under the conditions, for example, of temperature: 250 to 350°C., preferably 300 to 350° C., and a duration of 15 to 120 minutes,preferably 60 to 120 minutes. The pre-annealing treatment improves thetransistor performance by increasing the on-current and field-effectmobility.

Next, an etch stopper layer 9 is formed to protect the surface of theoxide semiconductor layer 4 (the surface of the oxide semiconductor onthe side of S/D electrode). The etch stopper layer 9 is formed for thepurpose of preventing deterioration of transistor characteristics by thewet etching of source-drain (S/D) electrode 5. The oxide semiconductorlayer 4 could be damaged by the wet etching, generating defects on thesurface. The kind of the etch stopper layer 9 is not particularlylimited. An insulating film such as, for example, SiO₂ is used. The etchstopper layer 9 is formed by deposition and patterning for the purposeof protecting the surface of the channel.

Subsequently, patterning for forming electrode is carried out byphotolithography and dry etching in order to secure electrical contactof the oxide semiconductor layer 4 to a source-drain electrode 5 formedsuccessively.

Next, the source-drain electrode 5 may be formed. The kind of thesource-drain electrode 5 is not particularly limited, and those whichhave widely been used can be employed. For example, similar to the gateelectrode, metals such as Mo, Al and Cu or their alloys may be used.Alternatively, a pure Mo may be employed as in an Example explainedbelow.

The source-drain electrode 5 may be formed by, for example, a depositionof the metal thin film by magnetron sputtering, followed by patterningby photolithography and wet etching.

The source-drain electrode 5 may be alternatively formed by a depositionof the metal thin film by magnetron sputtering, followed by patterningby lift-off method. It is possible to fabricate the electrode without awet etching process in this method.

Then, a passivation film (an insulating film) 6 is formed on the oxidesemiconductor layer 4. The passivation film 6 may be formed by, forexample, a CVD method. The surface of the semiconductor layer 4 (thesurface of the oxide semiconductor layer on the side of S/D electrode)may easily become conductive due to plasma-induced damage by CVD(presumably because oxygen defects formed on the surface of the firstoxide semiconductor act as electron donors), and therefore, N₂O plasmairradiation may be carried out before the formation of the passivationlayer 6. The conditions described in the following literature may beemployed as the N₂O plasma irradiation conditions.

-   J. Park et al., Appl. Phys. Lett., 1993, 053505 (2008)

Then, by photolithography and dry etching, a contact hole 7 is formedthrough the passivation film 6, followed by a formation of a transparentconductive film 8. The kind of the transparent conductive film 8 is notparticularly limited, and there can be used those which have usuallybeen used such as ITO.

The present invention encompasses a display device having the TFTs asdescribed above. As the display device, a liquid crystal display, anorganic EL display, or the like are exemplified.

The present application claims the benefit of priority based on JapanesePatent Applications No. 2012-192666 and No. 2013-094087 filed on Aug.31, 2012 and Apr. 26, 2013, respectively. The entire contents of thespecification of the Japanese Patent Applications No. 2012-192666 andNo. 2013-094087 filed on Aug. 31, 2012 and Apr. 26, 2013, respectively,are incorporated herein by reference.

EXAMPLES

The present invention is described hereinafter more specifically by wayof Examples, but the present invention is not limited to the followingExamples. The present invention can be put into practice afterappropriate modifications or variations within a range meeting the gistdescribed above and below, all of which are included in the technicalscope of the present invention.

Example 1

Thin film transistors having a two-layer oxide semiconductor layer shownin FIG. 1 were fabricated as described below, and their stress stabilityand other characteristics were evaluated. It is noted here that atransparent conductive film 8 illustrated in FIG. 1 was not deposited inthe present Example.

First, a Mo thin film of 100 nm in thickness as a gate electrode 2 andSiO₂ film of 250 nm in thickness as a gate insulator film 3 weresuccessively deposited on a glass substrate 1 (“EAGLE 2000” availablefrom Corning Inc, having a diameter of 100 mm and a thickness of 0.7mm). The gate electrode 2 was deposited using a pure Mo sputteringtarget by a DC sputtering method under the conditions: depositiontemperature, room temperature; sputtering power density, 3.8 W/cm²;carrier gas, Ar; gas pressure, 2 mTorr; Ar gas flow rate, 20 sccm.

The gate insulator film 3 was formed by a plasma CVD method using amixed gas of SiH₄ and N₂O with a carrier gas. Specifically, singlelayers of the gate insulator film 3 were deposited on a round shapeelectrode of 8 inches in diameter (total area of 314 cm²) as anelectrode of an CVD apparatus by varying deposition temperature, plasmapower density, and gas flow ratio (volume ratio) as shown in Table 1.The gas pressure was fixed at 133 Pa (not shown in the table).

Subsequently, oxide semiconductor films on the side of the gateinsulator film, of various chemical compositions shown in Table 1 weredeposited to a film thickness of 10 nm by a sputtering method usingsputtering targets having chemical composition corresponding to each ofthe oxide semiconductor layer under the conditions shown below.

Sputtering apparatus: “CS-200” available from ULVAC, Inc.

Substrate temperature: room temperature

Gas pressure: 1 mTorr

Oxygen partial pressure: 100×O₂/(Ar+O₂)=4 volume %

Film formation power: 2.55 W/cm²

Next, oxide semiconductor films on the side of the source-drainelectrode, of various chemical compositions shown in Table 1 weredeposited to a film thickness of 30 nm by a sputtering method usingsputtering targets having chemical composition corresponding to each ofthe oxide semiconductor layer under the conditions shown below.

Sputtering apparatus: “CS-200” available from ULVAC, Inc.

Substrate temperature: room temperature

Gas pressure: 1 mTorr

Oxygen partial pressure: 100×O₂/(Ar+O₂)=4 volume %

Film formation power: 2.55 W/cm²

The respective contents of metal elements in the oxide semiconductorlayer on the side of the gate insulator film and the oxide semiconductoron the side of S/D electrode thus obtained were analyzed by an XPS(X-ray photoelectron spectroscopy) method. Specifically, aftersputtering to a depth of about 5 nm from the outermost surface by Arion, the analysis was carried out under the conditions described below.The measurement of the oxide thin film by the XPS method was carried outusing a sample having thin films each having a thickness of 100 nmformed on a Si substrate and respectively having the same compositionsas the oxide semiconductor film.

X-ray source: Al K_(α).

X-ray output: 350 W

Photoelectron take-off angle: 20°

After the oxide semiconductor layer 4 (the oxide semiconductor layer onthe side of the insulating film and the oxide semiconductor layer on theside of the source-drain electrode) was deposited as described above,patterning was carried out by photolithography and wet etching.“ITO-07N” available from Kanto Chemical Co., Inc., an oxalic acid-basedwet etchant solution for oxide semiconductors, was used for the wetetching.

After patterning of each of the oxide semiconductor layer 4,pre-annealing treatment was carried out to improve the film quality ofthe oxide semiconductor layer. The pre-annealing was carried out at 350°C. under air atmosphere for 60 minutes.

Next, an etch stopper layer 9 consisting of SiO₂ was formed in athickness of 100 nm to protect the surface of oxide semiconductor layer.The formation of the film was carried out by a plasma CVD method using“PD-220NL” available from SAMCO Inc. In this Example, a mixed gas of N₂Oand SiH₄ diluted in nitrogen carrier gas was used for the formation ofthe SiO₂ film under the conditions shown below.

Film formation temperature: 230° C.

Gas pressure: 133 Pa

Film formation power density: 1.1 W/cm²

Flow rate ratio (in volume ratio) of SiH₄/N₂O: 0.04.

Then, for the purpose of electrically contacting the oxide semiconductorlayer 4 to the source-drain electrode 5, the etch stopper layer 9 thusobtained was subjected to patterning by photolithography and subsequentreactive ion etching (RIE).

Next, a pure Mo film was deposited as the source-drain electrode 5 by DCsputtering. Specifically, as for the gate electrode, a Mo thin film wasdeposited in a thickness of 100 nm. Subsequently, the source-drainelectrode was patterned through photolithography.

After the source-drain electrode 5 was formed as described above, apassivation film 6 was formed to protect the oxide semiconductor layer4. As the passivation film 6, a laminate film (250 nm in totalthickness) of SiO₂ (100 nm in thickness) and SiN (150 nm in thickness)was used. The SiO₂ and SiN films were formed by plasma CVD method using“PD-220NL” available from SAMCO Inc. In the present Example, the SiO₂film and the SiN film were successively formed. A mixed gas of N₂O andSiH₄ was used for the deposition of the SiO₂ film, and a mixed gas ofSiH₄, N₂, and NH₃ was used for the deposition of the SiN film. In bothcases, the film deposition power density was set to 0.32 W/cm², and thefilm deposition temperature was set to 150° C.

Then, a contact hole to be used for probing to evaluate transistorcharacteristics was formed in the passivation film 6 by photolithographyand dry etching. Thus, TFTs as shown in FIG. 1 were prepared.

Each of the TFTs obtained as described above was subjected to evaluationof stress stability as follows.

(1) Evaluation of Stress Stability Under Negative Biasing (NBTS)

In the present Example, stress biasing test was carried out by applyingnegative bias onto the gate electrode. The stress biasing conditions areas follows.

Source voltage: 0V

Drain voltage: 10 V

Gate voltage: −20 V

Substrate temperature: 60° C.

Stress application time: 2 hours

In the present Example, variation of threshold voltage during the stressbiasing test for a period of 2 hours was defined as the thresholdvoltage shift ΔV_(th), and TFTs having a ΔVth of smaller than 5.0 V inNBTS were categorized as “pass”.

(2) Evaluation of Stress Stability Under Light Irradiation and NegativeBiasing (LNBTS)

In the present Example, stress biasing test was carried out by applyingnegative bias onto the gate electrode while irradiating white lightsimulating a stress environment of actual liquid crystal display. Thestress biasing conditions are as described below. A white LED is usedfor the light source simulating a back light for a liquid crystaldisplay device.

Source voltage: 0V

Drain voltage: 10 V

Gate voltage: −20 V

Substrate temperature: 60° C.

Stress application time: 2 hours

Light source: white LED (LXHL-PW01 available from PHILIPS N.V.) 25000nit

In the present Example, variation of threshold voltage during the stressbiasing test for a period of 2 hours was defined as the thresholdvoltage shift ΔV_(th), and TFTs having a ΔV_(th) of smaller than 5.0 Vin LNBTS were categorized as “pass”.

(3) Measurement of SS Value

SS value is a minimum value of a gate voltage needed to increase thedrain current by one digit. In the present Example TFTs having a SSvalue of smaller than 0.55 V/decade in the LNBTS stress test werecategorized as “pass”.

(4) Measurement of on-Current (I_(on))

On-current (I_(on)) is a value of drain current measured at the gatebias of 30 V when the transistor is on-state. In the present Example,the on-currents were measured before and after the (2) LNBTS stresstest, and when their difference in absolute value (ΔI_(on)) is smallerthan 10%, the TFT was categorized “A” or “pass”, while those havingΔI_(on) of 10% or larger were categorized “B” or “fail”.

The results are summarized in Table 1. Each gas flow ratio (volumeratio) in the Table indicates ratio of SiH₄ where the flow rate of N₂Ois 100.

The rightmost column in each of the tables is for total evaluation, andeach sample was rated “A” if it satisfies all of the criteria explainedabove, and rated “B” if it fails to satisfy at least one of thecriteria.

TABLE 1 Metal element ratio in Metal element ratio in oxidesemiconductor on the oxide semiconductor on the Conditions for formingside of S/D electrode side of gate insulator film the gate insulatorfilm In Ga Zn Sn In Zn Sn Input Gas flow (atom- (atom- (atom- (atom-(atom- (atom- (atom- Temp. power ratio No. ic %) ic %) ic %) ic %) ic %)ic %) ic %) (° C.) (W) (W/cm²) SiH₄ N₂O 1 12.0 40.0 34.0 14.0 20 56.723.3 320 300 0.96 2.24 100 2 12.0 40.0 34.0 14.0 20 56.7 23.3 320 3000.96 4 100 3 12.0 40.0 34.0 14.0 20 56.7 23.3 300 300 0.96 4 100 4 12.040.0 34.0 14.0 30 56.7 23.3 250 300 0.96 4 100 5 12.0 40.0 34.0 14.0 3051.7 18.3 300 250 0.80 4 100 6 12.0 40.0 34.0 14.0 30 51.7 18.3 300 1000.32 4 100 7 11.0 45.0 31.2 12.8 20 56.7 23.3 320 300 0.96 2.24 100 811.0 45.0 31.2 12.8 20 56.7 23.3 320 300 0.96 4 100 9 11.0 45.0 31.212.8 30 51.7 18.3 300 300 0.96 4 100 10 11.0 45.0 31.2 12.8 30 51.7 18.3250 300 0.96 4 100 11 10.0 50.0 28.4 11.7 20 56.7 23.3 300 250 0.80 4100 12 10.0 50.0 28.4 11.7 30 51.7 18.3 300 100 0.32 4 100 13 17.0 17.047.0 19.0 20 56.7 23.3 300 300 0.96 4 100 Hydrogen content in LNBTS NBTSgate insulator film ΔV_(th) ΔI_(on) SS value ΔVth No. (atomic %) (V) (A)(V/decade) (V) Evaluation 1 1.2 0.5 A 0.30 0.3 A 2 1.2 0.5 A 0.28 0.3 A3 1.5 1.0 A 0.33 0.5 A 4 3.0 4.0 A 0.40 1.0 A 5 3.5 4.5 A 0.45 1.3 A 68.0 10.0 B 0.80 5.0 B 7 1.2 0.5 A 0.30 0.3 A 8 1.2 0.5 A 0.28 0.3 A 91.5 2.8 A 0.33 0.5 A 10 3.0 3.8 A 0.40 1.0 A 11 3.5 4.5 A 0.45 1.3 A 128.0 9.0 B 0.80 5.0 B 13 1.5 3.5 A 0.45 3.0 A

The results shown in Table 1 may be analyzed as follows.

Shown in Table 1 are results of the evaluation conducted for samplesprepared having IZTO as the oxide semiconductor layer on the side of theinsulating film and IGZTO as the oxide semiconductor layer on the sideof S/D electrode in various ratios of metal elements constituting theoxide semiconductor layer and a variety of deposition conditionsincluding deposition temperature, plasma power density, and gas flowratio for the gate insulator film.

Consequently, excellent properties were secured in every stress test asthe hydrogen concentration in the gate insulator film is lowered to thespecified range in the examples (Nos. 1 to 5, 7 to 11, and 27) for whichthe deposition conditions of the gate insulator film were controlled to;deposition temperature of 250° C. or higher, plasma power density of 0.7W/cm² or higher, and gas flow rate ratio (SiH₄/N₂O) of 0.04 or lower. Inaddition these samples showed high mobility of 6 cm²/Vs or higher(Mobility for each sample is not shown in the tables).

On the contrary, when the plasma power density for the formation of thegate insulator film was out of the preferred conditions, desirableproperties in terms of stress stability were not secured (Nos. 6 and12).

EXPLANATION OF REFERENCE NUMERALS

-   -   1 Substrate    -   2 Gate electrode    -   3 Gate insulator film    -   4 Oxide semiconductor layer    -   5 Source-drain electrode    -   6 Passivation film (insulating film)    -   7 Contact hole    -   8 Transparent conductive film    -   9 Etch stopper layer

The invention claimed is:
 1. A thin film transistor, comprising: a gateelectrode, an oxide semiconductor layer comprising at least a first anda second oxide semiconductor layer, an etch stopper layer, asource-drain electrode, and a gate insulator film, wherein the oxidesemiconductor layer is configured to be a channel layer; the etchstopper layer is configured to protect a surface of the oxidesemiconductor layer; the gate insulator film is interposed between thegate electrode and the channel layer; the first oxide semiconductorlayer is in direct contact to the gate insulator film and satisfies thefollowing expressions: 15≦[In]≦35, 50≦[Zn]≦60, 15≦[Sn]≦30 where [In],[Zn], and [Sn] represent a content of each element in atomic % relativeto a total content of all metal elements in the first oxidesemiconductor layer; a hydrogen concentration in the gate insulator filmin direct contact to the first oxide semiconductor layer is 4 atomic %or lower; and the second oxide semiconductor layer is in direct contactto the source-drain electrode and satisfies the following expressions:10≦[In]≦20, 30≦[Zn]≦40, 5≦[Sn]≦15, and 35≦[Ga]≦50 where [In], [Zn],[Sn], and [Ga] represent a content of each element in atomic % relativeto a total content of all metal elements in the second oxidesemiconductor layer.
 2. The thin film transistor according to claim 1,wherein the gate insulator film is a single layer structure or alaminate structure comprising two or more layers; in case of thelaminate structure, the hydrogen concentration is 4 atomic % or less ina layer which is in direct contact to the first oxide semiconductorlayer.
 3. A display device, comprising the thin film transistoraccording to claim
 1. 4. The thin film transistor according to claim 1,wherein the hydrogen concentration in the gate insulator film in directcontact to the first oxide semiconductor layer is 3.5 atomic % or lower.5. The thin film transistor according to claim 1, wherein the hydrogenconcentration in the gate insulator film in direct contact to the firstoxide semiconductor layer is 3 atomic % or lower.
 6. The thin filmtransistor according to claim 1, wherein the hydrogen concentration inthe gate insulator film in direct contact to the first oxidesemiconductor layer is 0.667 atomic % or more.